Method and system for characterizing color display monitor output

ABSTRACT

The present invention provides a technique for modeling the tone reproduction curve of a flat panel screen such as an LCD. The technique employs a second order function to predict the luminance output as a function of the input voltage based upon two known input values. This abstract is provided for the sole purpose of complying with the rules requiring an abstract to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure contained herein. This abstract is submitted with the express understanding that it will not be used to interpret or to limit the scope or the meaning of the claims.

RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 09/439,482 filed Nov. 12, 1999, which is acontinuation-in-part of U.S. patent application Ser. No. 09/422,215filed Oct. 19, 1999 and now U.S. Pat. No. 6,693,647, and claims thepriority of provisional application Ser. No. 60/108,444 filed Nov. 13,1998, Ser. No. 60/108,442 filed Nov. 13, 1998, and Ser. No. 60/108,229filed Nov. 13, 1998. This application further claims the priority ofprovisional application Ser. No. 60/191,908 filed Mar. 24, 2000.

BACKGROUND

1. Field of the Invention

This invention relates to display color correction and more specificallyto a method that can be used for characterizing flat panel displays, CRTdisplays, as well as other types of displays to permit color correctionof images.

2. Background of the Invention

Flat panel displays do not follow the tone reproduction curves (TRC)common to cathode ray tube (CRT) displays. Characterizing the TRC of adisplay is a first step of one method of correcting the color of imageson the display.

What is needed is a technique for characterizing the performance of flatpanel displays.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a technique forcharacterizing flat panel displays. With greater particularity, theinvention provides a method for predicting the color output of a displaymonitor as a second order function of the input voltage to the monitorby obtaining the input voltage value required to display on the monitoreach of at least two images having known luminance values, and solvingthe function as at least two simultaneous equations based on theobtained input voltage values to calculate the two powers for thefunction to describe the monitor tone reproduction curve.

In a further aspect, the present invention provides a method fordisplaying a color image on a display monitor by modeling the monitortone reproduction curve as described above, and adjustingcharacteristics of the image in accordance with differences between themonitor tone reproduction curve and a predetermined tone reproductioncurve.

In a still further aspect, the present invention provides a method forpredicting the color output of various types of monitors including LCDsand CRTs with the same second order function. In this aspect, the tonereproduction curve of a CRT can be modeled by setting one of the powersof the function equal to one.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a stylized block diagram of a network according to the presentinvention.

FIG. 2 is a flow chart of the process of the present invention.

FIG. 3A is block diagram of a network according to the presentinvention.

FIG. 3B is a flow chart of the process of the present invention.

FIG. 4 is block diagram of a network according to the present invention.

FIG. 5A is a flow chart of the process of the present invention.

FIG. 5B is block diagram of a network according to the presentinvention.

FIG. 6A is a screen view of a web page according to the presentinvention.

FIG. 6B is block diagram of an HTML file according to the presentinvention.

FIG. 7 is a block diagram of the process steps of the present invention.

FIG. 8 is a block diagram of the process steps of the present invention.

FIG. 9A is an enlarged view of an indicator according to the presentinvention.

FIG. 9B is an enlarged view of an alternate indicator according to thepresent invention.

FIG. 10 is a block diagram of an alternate embodiment according to thepresent invention.

FIG. 11 is a block diagram of an alternate network according to thepresent invention.

FIG. 12 is graph of a parameter space according to the presentinvention.

FIG. 13 is a parameter distribution curve according to the presentinvention.

FIG. 14 is a graph of display transfer functions according to thepresent invention.

FIG. 15 is a block diagram of a first alternate process according to thepresent invention.

FIG. 16 is a block diagram of a second alternate process according tothe present invention.

FIG. 17 is a detailed block diagram of a network according to thepresent invention.

FIG. 18 is a detailed transform curve according to the presentinvention.

FIG. 19 is a detailed diagram of a display screen according to thepresent invention.

FIG. 20A is a diagram of a guardian cookie redirection according to thepresent invention.

FIG. 20B is a diagram of a guardian cookie cleanup according to thepresent invention.

FIG. 21 is a graph of the display curves for a plurality of various flatpanel devices.

The features and advantages of this invention will become apparent fromthe detailed description and accompanying figures that follow. In thefigures and description, numerals indicate the various features of theinvention, like numerals referring to like features throughout both thedrawings and the description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, system 10 according to the present inventionprovides color images from network servers to users enhanced whenpossible with user specific color correction information to provide highfidelity color images to the users. In particular, in accordance with apreferred embodiment of the present invention, color server 20 mayprovide color catalog pages for clothing or other products to apotential buyer, such as user 12, adjusted to provide high fidelitycolor images in accordance with the color display characteristics ofdisplay 22.

In general, system 10 may include one or more network servers and one ormore users. Network servers may include color server 20, commercialserver 18, and server 76. Users may include users 12, 14 and 16,interconnected to network servers using network 13. Network nodes suchas color server 20 may serve as a user or client for some purposes and aserver for others. System 10 does not require a static server,constantly functioning as a server, in all embodiments, additionally,servers may also be composed of multiple machines.

Network 13 may be any type of network such as a LAN, intranet or aninternet such as the World Wide Web (WWW). Network 13 may also utilizeany type of appropriate network protocol, such as HTTP as used on theWorld Wide Web. Color server 20 may be used to host color correctableimages 50 to be made available to users of commercial or other networksites.

User 12 may be any conventional network client device and may includeone or more electronic devices 24, conventionally a personal computer orworkstation, and one or more display devices 22, conventionally a CRT orLCD display monitor. User 12 may also include remote storage 26 and/orlocal storage 28 within electronic device 24. Remote storage 26 may alsobe available to electronic device 24 through network 13. User 12 mayalso include one or more output devices 30 which may be any type ofprinter, recorder or plotter. User 12 may also include one or more inputdevices 32 which may be any type of scanner, reader, image capturedevice or other data transfer device.

Delivery of accurate images according to the present invention beginswith image request 54 sent to commercial server 18 for the display ofimage 56 on monitor 22 as image 52. Image request 54 may originate withuser 12 or any network device such as server 76. Image request 54 may bean individual request for a specific image, graphic, drawing, renderingor similar data file or it may be part of a larger data request such asa web page request. Commercial server 18 may respond to image request 54by then inquiring of the source of the image request to determine ifdisplay calibration or characterization data 38 for display 22 isavailable.

If display calibration or characterization data 38 is available tocommercial server 18, a color corrected version of image 56 may beprovided to user 12 in accordance with data 38. Thus, image 52 as thendisplayed on display 22 may be a more accurate color representation of areference or author image, image 56 than may otherwise be achieved.Image 56 may be corrected from any conventional format including but notlimited to rendering formats such as PCL and PDF, image formats such asJPEG 2000, AVI, MPEG 2, MPEG3, MPEG4, Quick time, Real Media, VRML, ART,WMF, FPX, BMP, PCX, TIFF, GIF, flash, or postscript.

Concurrent with delivery of color corrected images, display 22 maypresent a visual or other indicator 58, indicating that the image orimages being viewed are color corrected and accurate. Indicator 58, or avariation thereof, may also be used to indicate when images are notcolor corrected and/or provide other information to user 12, a networkserver or a network administrator. An online shopper or other user mayhave increased confidence to make purchases, as a result of viewingimage 52 over network 13, knowing the color of image 52 as actuallyviewed is accurate.

If display calibration or characterization data 38 is not available tocommercial server 18, user 12 may be invited to calibrate orcharacterize display 22 through network 13 with or without requiringplug-ins or downloads. Calibration may be accomplished from any networkserver 18 or from color server 20 or from a local agent 12A. Withoutdisplay calibration or characterization, image 52 may appear differentlyto users 12, 14 and 16 because of different operating systems, videocards, monitor settings and a range of other factors.

According to the present invention, process 131 as discussed below maybe a one-time process, involving images 62-65 and nine user interactionsthat may be mouse clicks, key presses, screen contacts or otherinteractive inputs to electronic device 24. Process 131 may includeother combinations or techniques to characterize a display system orcapture other personalization data. Process 131 may generally require 1to 2 minutes to complete, some circumstances may require more time.After completion of process 131, user 12 may receive color correctedimages without further setup. Discussions throughout that refer to colorcorrection should be understood to apply equally to gray scalecorrection. A characterizable and correctable network system accordingto the present invention may also be used to control delivery and ensurethe accuracy of sounds, smells, tastes and textures.

Commercial Element

Referring again to FIG. 1, according to the present invention agent 41may be implemented as image director 11 or as filter 23 resident oncommercial server 18. Filter 23 may modify the URL of an image elementof an HTML page according to the characterization of the display systemof user 12. Image director 11 may redirect the image request URLgenerated by the delivery of the requested HTML to user 12.

Data Block Sharing

Referring again to FIG. 1, calibration or characterization data 38 mustbe made available across multiple network domains for convenient use tocorrect and distribute images 40 or 42 across network 13. Some networkprotocols such as the HTTP protocol used on the WWW are able to storedata blocks on user 12 or other network devices. Data block 34 mayinclude many different types of information including, user preferencesand user hardware characteristics. Conventional techniques providingclient-resident data block storage are often referred to as providing“Cookies”. In addition, user cookie data may also be deposited on one ormore network machines for access by other network servers across thenetwork and to refresh user cookies should they become purged orotherwise unusable.

Cookie 36 may include one or more blocks of information passed from aserver and stored on a user, often as a result of the collection of thatinformation by the server from the user. Cookie 36 may then be used toprovide, or retrieve, information from a user to a server. For example,user 12 information concerning domain 77 may be passed from server 76 touser 12 and stored on user 12 as cookie 66. Subsequent connection ofuser 12 to server 76 would prompt server 76 to request cookie 66 toremind server 76 of information about user 12. This technique isconventionally used to provide personalized settings or informationspecific to user 12 on server 76 without requiring server 76 to storethe data information for all its users. For security purposes,conventional cookies are designed so that they cannot be shared acrossmultiple domains. Conventional cookies may even be limited to URL rangeswithin a domain, as is the case with the HTTP protocol. In aconventional network, a server in a first domain cannot access cookiesstored for another domain.

Conventional cookie techniques have not therefor be useful for providingdisplay characterization and/or calibration information about a user toa server unless the cookies are specific to that server, that is, unlessthe server has placed the cookies on the user. In accordance with thepresent invention however, various techniques of server and userredirection may be used to achieve results equivalent to sharing cookiesacross domains.

For example, if user 12 initiates request 60 to server 76, server 76 mayrequest data block 34 from user 12 to process request 60. Data block 34may include personal, preference, calibration and/or characterizationinformation related to user 12, as well as a time tag 34T or stale/freshtimer to permit synchronization of correction/characterization or otherinformation across the network. Data block 34 may also include index 34Ito database 46 permitting information 45 to be retrieved from database46. Other index information may also be included to permit regenerationof data blocks purged from a client machine.

Referring now to FIG. 2, a method of sharing data blocks according to afirst embodiment of the present invention begins at step 90 with request54 from a user 12. According to the present invention, users 12, 14 and16 may exist in one of three conditions. Standard condition 89S, inwhich no characterization and/or calibration has been performed,Correction Enabled condition 89C, in which characterization and/orcalibration has been performed according to the present invention,Modified condition 89M, in which characterization and/or calibration hasbeen performed not according to the present invention.

At step 91, agent 41 checks user 12 for a cookie 66.

At step 92 agent 41 determines if a cookie has been received. If nocookie is received, user 12 may be assigned a unique identifier ID andmay be redirected or bounced to color server 20 at step 93. Bouncing maybe accomplished using Java script or it may be accomplished using HTTPredirect or other suitable technique. A currently preferred embodimentof the present invention uses Java script.

If agent 41 receives cookie 66 from user 12, agent 41 and commercialserver 18 have enough information to provide user 12 with colorcorrected information at step 99A as requested in image request 54.

At step 94 color server 20 checks user 12 for a domain 15 cookie. If nodomain 15 cookie is present, user 12 is given global identifier GI andis bounced to color server 20 at step 95. The existence of uniqueidentifier ID signifies to agent 41 that user 12 is not characterizedand/or calibrated, and that corrected images may not be prepared foruser 12 using existing information.

One or more network servers 18 may include watchdog 18W to monitor thestatus of color server 20. If color server 20 is unavailable, time tag34T may be extended until color server 20 is available. If a user hasonly unique identifier ID user 12 may get a blank or marker cookie 34Buntil color server 20 is again available. Upon the return to service ofcolor server 20 the next interaction of a user with an extended time tag34T will update data block 34 and a user 12 with a blank or markercookie 34B will obtain a usable data block 34.

At step 96, if color server 20 detects a domain 15 cookie 66A in user12, user 12 is bounced to commercial server 18 along with displaycalibration or characterization data 38.

At step 97 agent 41 drops cookie 66C to user 12. Agent 41 uses thecontents of cookie 66 c to provide a corrected image 52 to user 12 atstep 98.

Referring again to FIG. 2, a method of sharing data blocks according toa second embodiment of the present invention begins at step 90 withrequest 54 from a user 12.

At step 91, agent 41 checks user 12 for a cookie 66.

At step 92 agent 41 determines if a cookie has been received. If no,user 12 is bounced to color server 20 at step 93.

If agent 41 receives cookie 66 from user 12, agent 41 and commercialserver 18 have enough information to provide user 12 with colorcorrected information at step 99A as requested in image request 54.

At step 94Q user 12 is bounced to commercial server 18 along with domain19 cookie 66Q. At step 95Q image request 54 is resent. At step 96Q,agent 41 detects domain 19 cookie 66Q. Commercial server 18 may use 66Qand image file 52F to provide user 12 with color corrected informationat step 99Q as requested in image request 54

Referring again to FIG. 2, a method of sharing data blocks according toa third embodiment of the present invention begins at step 90 withrequest 54 from user 12.

At step 91, agent 41 checks user 12 for a cookie 66.

At step 92 agent 41 determines if cookie 66 or information 92I has beenreceived. If cookie 66 is not present and information 92I is present,agent 41 becomes a user and requests characterization and/or calibrationinformation for user 12 from color server 20. Information 92I must beenough information to permit to color server 20 to recognize user 12 asthe beneficiary of the surrogate client action of agent 41.

If agent 41 receives display calibration or characterization data 38from color server 20, agent 41 drops cookie 66R to user 12. Using cookie66R, agent 41 and commercial server 18 have enough information toprovide user 12 with color corrected information at step 99A asrequested in image request 54.

Nodes connected to network 13 may include various combinations ofdisplays and electronic devices and may also include a variety of videohardware 68 and video software 70. Video hardware 68 may include videocards, boards, chips and accelerators. Video software 70 may includedrivers, applets and applications.

Display calibration and/or characterization data 38 does not exist foruser 14 in standard condition. Thus, user 14 may not receive colorcorrected images according to the present invention. Request 54 fromuser 14, requesting image file 52F from commercial server 18 will causeagent 41 to initiate examination 82. Examination 82 may be a request fora cookie or calibration and/or characterization data, and will not yieldany calibration and/or characterization data of any form from user 14.Agent 41 may be implemented as a software filter, an application or anyother suitable technique.

User 14 has no calibration and/or characterization data to return tocommercial server 18. Upon receiving no calibration and/orcharacterization data in response to examination 82, agent 41 maytransmit response 43 to user 14. Response 43 may cause user 14 totransmit request 31 to color server 20. Server 20 has no calibrationand/or characterization data to return and may transmit response 33 touser 14. Response 33 may include a unique identifier ID to identify user14 and cause commercial server 18 to drop a cookie 66E to user 14.Cookie 66E may be considered an empty cookie, it contains only uniqueidentifier ID and will not allow commercial server 18 to producecorrected images to user 14.

Alternatively, missing, inadequate, corrupted or otherwise unusablecalibration and/or characterization data from color server 20 mayinitiate inquiry 35 from color server 20 to user 14. Inquiry 35 may bean invitation or other initiation to user 14 to engage in remote orlocal calibration and/or characterization. If user 14 declines tocalibrate or characterize, image 52 displayed by user 14 would beuncorrected.

User 12 may be calibrated and/or characterized locally or remotely.Local calibration and/or characterization is discussed in U.S. Pat. No.5,638,117 to Engeldrum & Hilliard. Remote calibration and/orcharacterization is discussed in more detail below. After calibrationand/or characterization according to the present invention, displaycalibration or characterization data 38 may be stored locally on localstorage 28 of user 12 and/or stored remotely in database 46 on colorserver 20 or as data file 72. Calibration and/or characterization data38 may be stored as cookie 66, a block of data, or some similar methodusing other network protocols. Database 46 may exist only on colorserver 20 or may be parsed onto or duplicated on one or more networkmachines.

Request 54 from user 12, requesting image file 52F from commercialserver 18 will cause agent 41 to initiate examination 82. Examination 82may initiate return of cookie 66 to commercial server 18 if cookie 66was initially generated by an element within domain 19. Examination 82may also initiate return of display calibration or characterization data38 to commercial server 18. Return of either cookie 66 or displaycalibration or characterization data 38 may permit commercial server 18to correct image file 52F for display on display 22 as image 52.

If cookie 66 was deposited by a foreign domain and is inaccessible, ordisplay calibration or characterization data 38 is missing orinaccessible, examination 82 may return no data. Upon receiving nocalibration and/or characterization data in response to examination 82,agent 41 may transmit response 43 to user 12. Response 43 may cause user12 to transmit request 31 to color server 20. Request 31 may Colorserver 20 may transmit response 37 to user 12 which causes user 12 totransmit data 21 to commercial server 18. Data 21 may contain displaycalibration or characterization data 38 and/or other user profileinformation.

In modified condition, user 16 may have been calibrated and/orcharacterized locally or remotely to generate a foreign calibrationand/or characterization file 74. Foreign calibration or characterizationdata 74 may be stored locally in electronic device 78 or storedremotely. Calibration and/or characterization data 74 may be stored ascookie 80, a block of data, or some similar method using other networkprotocols. Agent 41 may detect foreign calibration and/orcharacterization file 74 or cookie 80. Upon detection of cookie 80 orforeign calibration and/or characterization file 74 agent 41 maytranslate the foreign files to translated data 84 to enable correctionof images according to the present invention. Alternatively, agent 41may also bounce user 16 to color server 20 along with translated data 84to enable color server 20 to drop translated data cookie 86 onto user16. Translation of foreign calibration and/or characterization file 74or cookie 80 may also be accomplished by color server 20.

The above process may be repeated as many times as necessary in order tosatisfy requests made of a server by a client.

The domains enumerated above need not be distinct from each other. Forexample, a domain that has a cookie it wishes to share and the domainthat distributes the cookie could be the same domain. Likewise, thedomain that has a cookie to share, the domain that distributes thecookie, and the domain that requests the cookie could all be the samedomain as well, data block sharing according to the present inventionmight be required if a domain and its cookies are partitioned by URLranges.

The act of sending the client from one domain to another in order toretrieve information may be done using any of a multiplicity of methodsincluding the use of a page description language including HTML or XML,by using some scripting language such as JavaScript or VBScript, or bysome combination of the above. For example, HTML tables using HTTP POSTor HTTP GET commands can be used in conjunction with JavaScript orVBScript to automate inter-page, and thus inter-domain, transfers.

Methods of supplying the information returned by a cookie sharing servermay include, but are not limited to, responses to forms, additional URLheader fields, or additional cookies in a URL's domain.

Guardian Cookies

Referring to FIGS. 20A and 20B, the process of redirecting a networkuser 500 from a network machine 502 to another network machine 504 toobtain images 506 and 508 according to the present invention mayinitiate multiple parallel image requests if image request 510 is for aweb page or other image composed of multiple discrete image files. As aresult of multiple image requests from an uncharacterized user 12multiple cookies or data blocks 34 may be deposited on user 12, eachdata block 34 having a different time tag 34T. In another embodiment ofthe present invention, guardian cookies 512 and 514 may be used to avoida user being assigned multiple unique identifier ID by each networkmachine.

For example, user 500 may be uncharacterized or simply unknown to bothnetwork machine 502 and network machine 504. Request 510 from user 500may generate multiple parallel image redirections 516 and 518. Imageredirections 516 and 518 may generate image requests 520 and 522respectively from user 500 to network machine 504. If requests 520 and522 do not include data block 34 network machine 504 may assign eachrequest a unique identification, thus request 520 may result in image506 being sent to user 500 along with a data block 34 including uniqueidentifier IDX. Request 522 may result in image 508 being sent to user500 along with a data block 34 including unique identifier IDY. The lastdata block to arrive at user 500 will overwrite previous data blocksthus for example data block 34 with IDX may be the last to arrive andthe data block to survive. Relative to network machine 504 user 500 hasretained unique identifier IDX.

Arrival of each image 506 and 508 and the associated data blockinitiates notices 524 and 526 respectively to network machine 504. Eachnotice includes the unique identifier which initiated it. Arrival ofnotice 524 and notice 526 causes network machine 502 to send guardiancookies 512 and 514 respectively as well as data cookies 528 and 530respectively to user 500, each guardian cookie including includes theunique identifier which initiated it. The last of data cookies 528 and530 to arrive at user 500 overwrites any previously saved cookies fromnetwork machine 502 for this example assume that data cookie 530 andunique identifier IDY overwrite data cookie 528 and unique identifierIDX. Thus user 500 includes data block 34 and IDX form network machine504 and data cookie 530 and IDY and guardian cookies 512 and 514.

As discussed elsewhere, upon expiration of time tag of data cookie 530user 500 may initiate a cookie refresh with network machines 502 and 504and the presence of guardian cookies 512 and 514 indicates that user 500may be in possession of multiple identifiers.

Referring now to FIG. 20B, expiration of timer 530T may be one ofseveral triggers that will prompt cookie refresh cycle 532. User 500 maytransfer data 534 to network machine 502 indicating the expiration oftimer 530T. Network machine 502 may poll user 500 and discover thepresence of more than one guardian cookie such as guardian cookies 512and 514 and that data cookie 530 and unique identifier IDY were the lastto arrive at user 500 and thus are the repositories of the data and IDrespectively for user 500. User 500 may then be redirected to transferto network machine 504 unique identifier IDY which may also beaccompanied by a request for a cookie refresh. Unique identifier IDY isone of several unique identifiers that were transferred to user 500 withthe parallel image requests that created the race condition, thus uniqueidentifier IDY is a recognized value therefor user 500 is alsorecognized. Network machine 504 drops updated cookie 536 which may alsocontain unique identifier IDY to user 500. Updated cookie 536 overwritesdata block 34 and overwrites unique identifier IDX with uniqueidentifier IDY. As a result both network machine 502 and network machine504 agree that user 500 is represented by unique identifier IDY and nowhas the latest data from network machine 504 in the form of updatedcookie 536. User 500 then transfers data from updated cookie 536 tonetwork machine 502 prompting network machine 502 to drop new cookie 538and guardian cookie 540 and unique identifier IDY. New cookie 538overwrites data cookie 530 and guardian cookie 540 overwrites guardiancookies 512 and 514. The presence of only one guardian cookie serves toindicate that both network machine agree on the ID of user 500.

Remote Characterization

Referring now to FIG. 3A, a user of a local computer 100 may desirecharacterization and/or calibration of one or more input/output devicessuch as display 102, scanner 104, other image input device 106, printer108, plotter 110, or other image output device 112. Computer 100 may beconnected via a wired or wireless network such as network 114 ordirectly via modem or cable or other means to a remote server 116 wheresoftware 118 and data 120 needed for characterization may be stored.

After link 122 is established between a Remote Server 116 and computer100, either server 116 or computer 100 may request characterizationand/or calibration service from a remote server on behalf of computer100. Server 116 may then initiate a characterization program 124.Characterization program 124 may send one or more characterizationimages 126 or test patterns to computer 100 and its associated devices102, 104, 106, 108, 110, and 112. If the device to be characterized isan output device such as display 102, printer 108, plotter 110 or imageoutput device 112, characterization or test image 126 may be presentedto a user or a local calibration mechanism 128 using computer 100'smanner of output onto the selected device.

If the device to be characterized is scanner 104, image capture device105 or other image input device 106, characterization or test image 126may be presented to the user or local calibration mechanism 128 using aconventional input from the device to be characterized and aconventional output onto display 102 or any other device.

Referring now to FIG. 3B, a process 131, of remotely characterizingdisplay 102 according to the present invention begins at step 130 with arequest 125 for characterization that may be initiated by computer 100or server 116. At step 132, based upon request 125, server 116 initiatescharacterization program 124. At step 134, characterization program 124through server 116 transmits image 126 or other test pattern which maythen be presented to the user or local calibration mechanism 128 ondisplay 102 or other device to be characterized. At step 136 a user orlocal calibration mechanism such as calibrator 128 may make one or morechoices based on the image or test pattern as it appears on display 102.Choices made by a user may be made in any conventional manner as throughkeyboard or mouse entry or any other suitable tactile feedback device, auser may also indicate their preferences in other ways such as verbally.At step 138 the choice or choices may result in choice data 150 or otherquantifiable data that may be captured locally and/or communicated backto characterization program 124 on server 116 for capture.

One of the choices to be made by a user may be to select a level ofthoroughness of the characterization and/or calibration.Characterization program 124 may provide one or more options for devicecharacterization including full or partial characterization, or multiplelevels of characterization complexity. At step 140 characterizationprogram 124 determines if a sufficient number of images or test patternshave been sent to computer 100, and if a sufficient number of responseshave been captured to complete the level of characterization desired. Inanother aspect of the present invention characterization program 124 mayalso evaluate choice data 150 to determine if sufficient data has beenreceived to adequately characterize computer 100 at the desired level.If insufficient data has been captured characterization program 124 mayrepeat process 131 from step 134 until sufficient choice data has beencaptured.

After choice data 150 has been transmitted to server 116, choice data150 may be used by characterization program 124 or other electronicalgorithm to create characterization file 152 about the device to becharacterized.

Characterization file 152 might be used for one or more of the followingapplications:

-   -   a) send characterization file 152 to computer 100 for local        usage including, but not limited to, providing operating system        101 of computer 100 with information about the color        capabilities of computer 100; and/or    -   b) subsequently use characterization file 152 or other        characterization information for modifying or otherwise        controlling the flow of images such as still image 154 or        streaming images 156 for display, output or other use by        computer 100 based on the contents of characterization file 152;        and/or    -   c) store characterization file 152 or other characterization        information locally on a network node such as server 116 or        other computers connected to server 116; and/or    -   d) send characterization file 152 or other characterization        information to a third location such as server 158; and/or    -   e) feed into creation or alteration of the test patterns,        images, or other calibration and characterization implement such        as image 126; and/or    -   f) otherwise provide characterization file 152 or other        characterization information for use by software 118, other        programs, or other devices in providing images or other services        to computer 100.

Referring now to FIG. 4, in another aspect, the present invention mayinclude a combination of client software 160 and server software 162connected using network 164 and using suitable network protocols such asInternet protocols 166. It is expected that many individual localcomputers such as computer 168 may from time to time connect to any of anumber of remote servers such as server 170 over a network such asnetwork 164 which may be the Internet. At computer 168 with display 172as the device to be characterized, a user may initiate a request such asrequest 174 to server 170. Server 170 may incorporate images, data, testpatterns, and/or logic embodied in onto an appropriate hardware platform178. Program 176 or other suitable characterization programs may do oneor more of the following:

-   -   (a) manage communication link 180 with computer 168,    -   (b) select one or more appropriate characterization images        and/or test patterns or other test data such as image 182 to be        sent to the device to be calibrated. The selection of        appropriate test images may be determined by the level of        complexity of characterization desired, by the hardware to be        characterized, by the characteristics of the connection, or by        characteristics of images to be displayed.    -   (c) create, change or alter existing calibration images or test        pattern to send and/or change the order thereof if required,    -   (d) send one or more calibration images and/or test patterns,    -   (e) collect characterization and/or calibration data such as        choice data 184 returned from computer 168,    -   (f) create characterization information such as characterization        file 186 from analysis of the images or test patterns such as        image 182 sent and from the responses such as choice data 184        received,    -   (g) store characterization file 186 on server 170 and/or        connected machines such as server 171,    -   (h) use characterization file 186 to modify images such as image        190 or to change the flow of unmodified images such as images        192 sent to computer 168    -   (i) transmit characterization file 186 to other sites such as        site 194 for use at those sites to provide services such as data        196, which may include programs, data and/or images, to computer        168 or other purposes, and/or    -   (j) transmit characterization file 186 to computer 168 for local        usage.

For example, the present invention might be used as a technique tocharacterize client monitors over the Internet and to use thecharacterization information to color correct images sent to that clientso as to provide accurate color display over the Internet.

Page Title Signaling

In a still further embodiment, the present invention enables a serverapplication to signal a client application or hardware outside of normalbrowser communication channels such as a dead drop. Thus a clientapplication may monitor URLs arriving at the client browser and anencoded message in an arriving URL may be used to trigger a clientapplication to perform a predetermined action or actions. In addition todead drop signals, a URL may have encoded information to trigger thebrowser or other client application to perform one or more of manyactions such as modify color depth. A URL may also include many otherencoded information such as subset parameters or other client or serverinformation.

Correction Notification

In another aspect of the present invention, computer 168 may be providedwith icon 173 or other suitable notification to indicate the colorcorrection status of images on display 172. Display 172 may be aconventional CRT or other suitable image display device such as LCD,flat panel, digital ink, or printer to paper or film. Informationdescribing or notifying a user or other element of a network about therelative or absolute condition of an image is critical since the enduser is often in a remote location, separated in time and distance fromthe author of the image or images, and unable to know thecharacteristics of the image or images being viewed. In particular, thepresent invention may automatically inform viewers and/or otherreceivers of digital images as to the state of color correction for theimages, or one or more of the color metric states such as white point orgamma or others, thus notifying a viewer of the visual integrity of theimage being displayed. Consequently, viewers may feel assured and secureabout images they see as to the accuracy of those images.

Image status 183 or accuracy of image 182 may be determined relative toan authoring image and may include one or more image characteristics ormetrics 181 such as white point, gamma, black point, luminance or othersuitable characteristic. Image 182 may be either digital or analog.Alternatively, image status 183 of image 182 may be determined as anabsolute or relative value.

In particular, the present invention may be implemented as a softwareprocess 185 that may be a stand alone application or it may be loadedinto either an Internet browser or server technology. Alternatively thepresent application may be implemented as a hardware or softwarefunction of the operating system, or it may be a strictly localapplication such as on a photo CD. A browser is a client applicationthat enables a user to view HTML (or equivalent) documents on the WorldWide Web, another network, or the user's computer. The software may beimplemented in the form of a set of executable code such as a smallprogram or an applet, including Java or ActiveX application programs,that may be loaded into a web browser, such as Microsoft's InternetExplorer or Netscape's Navigator or other suitable application. Thesoftware may also be implemented on server 170. The present inventionmay be incorporated in server code such as Cosmo Color from SiliconGraphics or other suitable application. One skilled in the art willrecognize that other conventional or newly developed software processesmay be used as well and the invention may be implemented using hardwareor a combination of hardware and software. One skilled in the art willrecognize that the invention can apply to other browser technology, suchas local CD browsers and other non-Internet browsers and may use HTML orother markup languages such as but not limited to XML/XSL, XGML orDHTML.

Referring to FIGS. 5A and 5B, a flowchart of process 240 forimplementing the present invention through a sample network 242 isillustrated. For example, using Internet protocols, the presentinvention is typically enabled when browser 244 begins to reassemble webpage 246 on display 248, following the hidden HTML codes or othersuitable protocols in web page 246 to determine where to place one ormore elements such as element 252 which may be text, images, graphics orvideos onscreen. In particular, algorithm 256 may be implemented whenbrowser 244 begins to assemble element 252 or other part of a requestedpage. One skilled in the art, however, will recognize thatimplementation of the present invention can be initiated at anytime apage element requiring accurate color or gray scale including a graphic,image or video is present. Color or gray scale accuracy is identifiedhere as high fidelity or identical rendition of a page element ascompared to the image of the page element as viewed on the authoringdisplay, or as an absolute within a color space.

The technique according to the present invention initially determineswhether the image has been color enabled as shown at step 241 andsubsequently whether a user such as client 250 has been colorcharacterized or corrected as shown at step 243. To detect whether animage is color enabled according to the present invention, an algorithmsuch as algorithm 256 may detect whether color correction informationsuch as color specific files 258 or registry entries 260 are associatedwith a page element such as element 252. Color correction informationmay also include: (1) user specific Hypertext Markup Language (HTML)tags within the web page that designate the color properties of thesource image such as tags 262, other markup languages such as XML, XSL,XGML or DHTML may also be used, (2) a color profile 264 which may be astandard profile such as ICC, color sync, SRGB or SRGB64 embedded withinthe image file itself and (3) pointers to user specific (i.e. HTML) orstandard (i.e. ICC profiles) color files associated with the image filesuch as color specific files 258. At step 245, algorithm 256 maydetermine whether network 242 is acting in accordance with steps 241 and243 above to provide a faithful rendition of element 252.

Upon determining whether the image is color enabled at step 241 andwhether client 250 has been color characterized at step 243,notification element 254 may be provided as an indication of the statusor fidelity of element 252 currently being viewed by the client. Inparticular, at step 245 when an image such as element 252 is colorenabled and corrected, notification may be provided to a client such asclient 250 that the color of the image is accurate. If the image is notcolor enabled, at step 247 notification may be provided to the clientthat the color of the image may not be accurate. If the client is notcolor characterized or calibrated, at step 249 notification may beprovided to the client that the color of the image may not be accurate.Notification steps 247 and 249 may result in the same indication toclient 250 or distinct notifications may be used. Alternatively,notification may be provided to another server, network administrator orother interested device. After notification of client 250 at eithersteps 245, 247 or 249, algorithm 256 may enter a standby mode untilanother web page with image elements is detected. Notification element254 may be a part of web page 246 delivered from a network server ornotification element 254 may be generated on device 259 for display ondisplay 248.

Notification may include many variations, one or more icons may be usedas well as variations of the image in question. Different cursors may beused to provide notification as well as changes to the users interfacecharacteristics “skins”. Notifications may be provided in a conventionalWindows icon tray, or adjacent the image on the image or elsewhere onthe display.

In a currently preferred embodiment of the present invention algorithm256 may detect whether a web page such as web page 246 includespredetermined HTML tags such as tags 262. For example, when a web pagewith an image is color enabled, the HTML tags direct a browser todisplay a predetermined text as a headline of a certain size, such asthe title “True Internet Color™”.

Referring now to FIGS. 6A and 6B, a screen view of a web page 266 havinga title “True Internet Color™”, (True Internet Color™ is a trademark ofE-color Inc.) in title bar 264 and HTML file 270 that created it areshown. The presence of indicator 268 such as “True Internet Color” in atag such as tags 262 may enable algorithm 256 to recognize that imageson page 266 are color enabled. Thus when a web page includes the title“True Internet Color™” the image is considered to be color enabled. Thepresent invention is not limited to recognition of HTML tags directed atthe title “True Internet Color™,” but rather, indicator 268 may use anypredetermined tag configuration such as HTML tag, or web image tagconfiguration.

<html> Marks the beginning of an HTML-coded file <head> Marks the startof the header section and may contain descriptive information notdisplayed onscreen such as the title and author. It may also holdsformatting information, such as style sheets. <title>Shop-o-rama TrueInternet Color(r)</title> Sets the web page's title, displayed in theblue bar atop the screen. This also affects the displaying window'sexternally viewable and/or detectable attributes. </head> Marks the endof the header section and may contain descriptive information notdisplayed onscreen such as the title and author. It may also holdsformatting information, such as style sheets. </html> Marks the end ofan HTML-coded file

To determine whether an image such as page element 252 is color enabledvia ICC color correction information, a system according to the presentinvention such may detect whether ICC profiles (for the devicecharacteristics of the reference image as represented on the referencedevice) are embedded within an image file, such as element 252, basedupon an ICC profile format specification. In particular, the presentinvention may detect data 253 stored in ICC profiles such as profiles255, which are described in the ICC profile specification. ICC profilessuch as profiles 255 are device profiles that can be used in pairs totranslate color data created on one device such as device 257 into anative color space C of another device such as device 259. Morespecifically, an ICC profile such as profile 261 may be provided foreach device such as device 257 and may be used according to the presentinvention to transform color image data such as element 252 from adevice-dependent color space to the profile connection space, and totransform color image data from the profile connection space to anotherdevice-dependent color space. ICC profiles such as profiles 255 for thedevice characteristics of the reference image as represented on thereference device may be embedded in the image file such as element 252or stored in a memory in a connected computer such as device 259. Forexample, the ICC profiles could be stored in a memory, accessible by aCPU, and associated with the image instead of embedded. Additionally, itshould be noted that ICC profile can be accessed by the client from avariety of other sources such as network interface or from otherexternal devices via a modem interface.

To determine whether an image is color enabled-even without an embeddedor associated color profile a system according to the present inventionmay detect whether the image is in a known color space, such as sRGB.sRGB is a well-defined color space, includes various versions such assRGB 64, and is further defined at http://www.srgb.com. One skilled inthe art will recognize that implementation of the present invention maybe used with any kinds of images, including but not limited to thosesubject to compression techniques, such as GIF, PNG or JPEG formattedimages.

Referring to step 243, the present invention interrogates the clientsystem to determine if that system is characterized and calibrated tothe same state, or to a different but known state. In other words, thepresent invention detects the presence of a transfer function in theclient system, i.e. in the hardware or software (or the combination ofhardware/software and human perception). In particular, the presentinvention checks file entries and registries, or pointers to suchentries and registries, to determine whether characterization parametersare present. A flag, initialized to a set value, signals whether theclient system has been characterized. For example, in a typicalembodiment, a binary flag initialized to a zero value is set to anon-zero value when the present invention detects the client system ischaracterized. In accordance with the present invention, a client mayuse any type of conventional or newly developed color calibration systemincluding, for example, the interactive color calibration methoddisclosed in U.S. Pat. No. 5,638,117.

Referring to step 16, the present invention then determines the whetherthe system is acting in accordance with steps 12 and 14 above to providecolor accuracy. In particular, once the present invention confirms thatthe presence of color correction information in the displayed image(step 12) and the image has been adjusted, as needed, to displayproperly on the calibrated or characterized client system (step 14)(i.e. color accuracy is being provided for in step 16), a notificationis displayed to the user (step 18). When the software process determinesthat color accurate display is occurring on all or part of the image,then an appropriate notification is made to inform the user that colorcorrection has occurred where marked. One skilled in the art willrecognize that the particular type of notification is not critical tothe invention. The notification may be visual or non-visual notification(e.g. audio). For example, the visual notification may be an icon thatprovides users with a visible indication about the integrity of colorimagery currently being viewed by the client at a specific web site. Itdoes this by briefly flashing the cursor for a fraction of a second toindicate if whether or not the page is being viewed utilizing colorcorrection. This icon can be implemented in addition, or instead, in theOS, in a web-enabled application, or in a browser (when implemented onas a client-side application); or it can be implemented as an image,tag, program, or watermark embedded within a web page by the web serveror by any of the links between server and client within the networkinfrastructure.

For example, when a user requests a Web page from a web site enabled bythe present invention, the HTML is sent to the client directly from theweb site's main servers. A specially attached URL link calls upcolor-corrected images from the hosted server, and the client's browsersintegrate the two pieces automatically. Thus, in accordance with thepresent invention, the notification not only provides notificationfeedback to the user, but also reinforces a message of data fidelity tothe end-user in determining whether the color data is accurate or not.The present invention has applicability for any client viewing ordisplay application where color accuracy is important to thecommunication of information. Examples include, but are not limited to,viewing artwork, fashion, cosmetic, logo or brand colors, paint,photography and other color-sensitive information over a medium such asthe Internet where content viewer and content creator are disconnectedby physical space and/or time. Although, for illustrative purposes, thepresent invention is described and illustrated utilizing web pageshosted on a server and displayed with color correction on a client, theinvention is not limited to such a configuration. Rather, the presentinvention would apply equally well to images displayed on any imagingperipheral including transmissive, reflective, and other source and/orclient imaging technologies. Moreover, the present invention would alsoapply to images not viewed by the Internet, such as images withincomputer applications, TV, broadcast, or other client output media ofany kind, including printed output. The present method would apply toboth digital images and analog images including both real and syntheticimages authored for, and/or viewed on, a client system.

The present invention may be implemented as a client-based notificationsystem 30 as shown in FIG. 7 or server-based notification system 50 asshown in FIG. 8. With respect to a client-based notification system, thepresent invention may be installed on a client system such as client 38,peripheral, and/or other output technology that has various states ofvisual display to notify the user about the state of color correctionfor digital images output or displayed. Referring to FIG. 7, afunctional block diagram of a client-based notification system 30 forproviding critical end user feedback as to the color correction statusof imagery on a client display is illustrated. Client-based notificationsystem 30 is shown with hosted color server 32, mirrored server 34,non-mirrored server 36 and clients 38, 40, 42, 44 and 46 which representthe various types of clients, that is, clients such as 38 and 40 whichinclude the client based notification techniques of the presentinvention (indicated by the term “icon”), clients 38, 40, 42 and 44which are characterized for color, clients 42 and 44 which have a knowntransfer function and client 46 which is not characterized, has no knowncolor transfer function and does not include a notification systemaccording to the present invention.

Icon 66 depicted in FIG. 9(b), provided by client 38 is preferablyinitiated to a non-corrected state. When client 38 sends a request tomirrored server 34, which mirrors hosted color server 32, a colorcorrected requested image is sent from hosted color server 32 to client38 including a color notification tag, such as a specific HTML title barflag. The Web page HTML from server 34 includes a color notification tagwithin its HTML tags to indicate in the title bar that the images to besent by server 32 have been enabled for color correction. For example,as shown in FIG. 6A, the title bar of the web page may include anotification in its title bar, such as “True Internet Color”, inaddition to other terms such as the name of a related company, toindicate color correction. As noted above, one skilled in the art willrecognize that the present invention is not limited to the detection ofpredetermined HTML title tags, rather, any device capable of detectionmay be used as the color notification tag. Upon arrival at the client38, the present invention detects the color notification tag byevaluating the HTML tags sent from server 34 to determine whether theimage delivered from server 32 has been color correction enabled bydetecting the True Internet Color tag in the title. It also checkswhether client 38 has been color characterized or calibrated to a knownstate. If both conditions are true, an icon such as icon 66 depicted inFIG. 9(b) is changed to a corrected state as depicted by icon 64 in FIG.9(a). In contrast, when client 38 sends a request to a site providingnon color corrected pages, such as non-mirrored server 36, which is doesnot include the special HTML tags, such as “True Internet Color” toindicate color correction, then icon 66 remains unchanged from itsnon-corrected default state.

Still referring to FIG. 7, local client 40 is characterized but includesno applet incorporating the present invention for notification asindicated by the phrase “no-icon”. Local client 40 is in directcommunication with mirrored server 34 and hosted color server 32. Localclient 40 provides no notification icon. When local client 40 sends arequest to mirrored server 34, the requested image is color correctedand sent back by hosted color server 32, with no notification icon onclient 40. When local client 40 communicates with non-mirrored server36, the requested image is not color corrected, and there is nonotification icon to this effect.

Client 42 is characterized and includes a known (characterized orcalibrated) transfer function but no applet incorporating the presentinvention for notification. Client 42 is in direct communication withmirrored server 34 and non-mirrored server 36 and in directcommunication with hosted color server 32 via redirection requests frommirrored server 34. Client 42 provides no notification icon. When client42 sends a request to mirrored server 34, the requested image iscorrected. However, no notification indicating color correction is sentback to client 42. When client 42 sends a request to non-mirrored server36, the requested image is not color corrected and no notification ofcolor correction is sent back to client 42. In such case, the title barof the web page would not indicate a color corrected image.

Client 44 is characterized and includes a known (characterized orcalibrated) transfer function and includes an applet incorporating thepresent invention for notification. Client 44 is in direct communicationwith mirrored server 34 and non-mirrored server 36 and in directcommunication with hosted color server 32 via redirection requests frommirrored server 34. Client 44 provides a notification icon. When client44 sends a request to mirrored server 34, the requested image sent byhosted color server 32 is color corrected. In such case, the title barof the web page would indicate a color corrected image. Notificationindicating color correction is sent back to client 44 indicating a colorcorrected image being displayed. When client 44 sends a request tonon-mirrored server 36, the requested image is not corrected and nonotification of color correction is sent back to client 44. In suchcase, the title bar of the web page would not indicate a color correctedimage.

Client 46 is neither characterized nor includes an applicationincorporating the present invention for notification. Client interactswith non-mirrored server 36 only and provides no notification icon. Whenclient 46 sends a request to non-mirrored server 36, which is not inmirror communication with hosted color server 32, the requested imagesent by non-mirrored server 36 is not color corrected and nonotification is provided to the client 46. In such case, the title barof the web page would not indicate a color corrected image.

Referring now to FIG. 8, a functional block diagram of a server-basednotification system 50 for providing critical end user feedback as tothe color correction status of imagery on a client display isillustrated. With respect to a server-based notification system, thepresent invention may be installed on a web site server to notify theuser about the state of color correction for digital images output ordisplayed. In particular, the icon of the present invention can beimplemented in an image, tag, program, or watermark embedded within aweb page by the web server or any of the links between server and clientwithin the network infrastructure. Server-based notification system 50is shown with hosted color server 52, mirrored server 54, non-mirroredserver 56 and clients 58, 60, 62 and 64. The icon of the presentinvention is installed in hosted color server 52 and mirrored server 54and not in non-mirrored server 56.

When client 58 sends a request to hosted 52, client 58 may communicatethrough some means that it is a client that is of a specific, knowncalibration. This notification may be included in the HTML stream sentby the browser, or via any other method. In that case, if a colorcorrected image is sent from hosted color server 52 (or from mirroredserver 54) to client 58, then an icon is also sent by mirrored server 54or by hosted color server 52 to indicate that the image has beencorrected. In contrast, when client 58 sends a request to non-mirroredserver 56, which is not in communication with hosted color server 52,non-mirrored server 56 does not include an icon (or sends an iconindicating that no color correction has occurred).

In accordance with an alternative embodiment of the invention, client 60is characterized and includes an applet incorporating the presentinvention for providing notification. Local client 60 is in directcommunication with mirrored server 54 or hosted color server 52, whichalso includes the notification icon. When client 60 sends a request tomirrored server 54 or hosted color server 52 as described above, therequested image is color corrected and sent back by web site serveralong with a notification icon indicating a corrected state. Mirroredserver 54 also sends the HTML tags indicating color correction and theicon on client is changed to indicate the corrected state. Logic isimplemented to arbitrate between the state of the two icons(server-based and client-based). For example, in one embodiment eitherthe server or client based notification icon may take precedence whilein another embodiment a third icon, similar to the icons shown in FIGS.5(a) and (b), may be used to indicate the presence of a different levelof color correction based on the presence of both server and clientbased notifications.

Client 62 is neither characterized nor includes an applicationincorporating the present invention for notification. When client 62sends a request to server 54, then either server 52 would use HTML tagsto add some watermark or other image to the web page to indicate coloraccuracy; or server 52 would request web server 54 to send an imagewhich already has an icon superimposed on the image sent by server 54.When client 62 sends a request to non-mirrored server 56, the requestedimage sent by non-mirrored server 56 is not color corrected and nonotification is provided to client 62. In such case, the title bar ofthe web page would not indicate a color corrected image, and no iconwould be sent by non-mirrored server 56.

In accordance with an alternative embodiment of the present invention,depending upon the relationship between the mirrored server 54 andhosted color server 52, hosted color server 52 may require mirroredserver 54 to identify images not color corrected. In such case, aserver-based icon can be sent to a client to indicate images which arenot color corrected.

In accordance with another alternative embodiment of the presentinvention, multilevel icon certifications may be provided. Inparticular, multilevel icon certifications can be utilized todistinguish between icon certifications between various entitiesproviding for color correction. For example, when hosted color server 52provides color correction, an icon identifying not only colorcorrection, but correction specifically provided by a particular hostedcolor server, is sent to the client. On the other hand, if colorcorrection is provided by another entity, an icon identifying colorcorrection, without identification of a specific entity providing forcorrection, is sent to the client.

Partial File Processing

To increase the speed of providing color corrected images to a user,commercial server 18 of FIG. 1 may store partially preprocessed datafiles such as image files or may partially preprocess data fileson-the-fly. Similarly, only that portion of a compressed image filenecessary to correct the color need be decompressed for color correctionthus expediting the process. In general, images available on network 200may conform to one or more compression standards to permit greaterthroughput of information and higher inter-connectivity. Severalstandard image formats such as JPEG (Joint Photographic Experts Group),or MPEG (Motion Picture Experts Group), or GIF (graphical interchangefile format) may be found on a network such as the Internet.

Referring now to FIG. 10 process 300 is a conventional technique forimage compression such as, for example, a JPEG format. Image 302 may beany image such as a line drawing, a black and white or color photograph,or any other image. Image 302 is compressed by compression device 304according to a compression standard, here JPEG standards, and results inJPEG file 306. A compressed file such as a JPEG file 306 may haveseveral identifiable elements, such as luminance element 308, colorelement 310, and miscellaneous elements 312 and 314. Miscellaneouselements such as element 312 may include information unnecessary for theultimate display of a color corrected images over a network, such as athumbnail image. Other compression standards may have different elementsand may function similarly for color spaces using differentspecification characteristics.

A compressed image file such as image file 316 may be partiallyuncompressed to expedite color correction as shown in FIG. 10. At step317, file filter 318 processes image file 316 to separate compressedluminance elements and compressed color elements such as compressedluminance element 316L and compressed color element 316C respectively.Unnecessary file elements such as miscellaneous elements 312 and 314 ofFIG. 10 may be discarded to expedite processing. Compressed colorelement 316C is passed along at step 319, as no processing of compressedcolor element 316C is required according to a currently preferredembodiment of the present invention. However, use of other color spacesor compression techniques may require some processing of a generallyunused element such as compressed color element 316C and may result inprocessed elements such as element 322.

At step 321 one or more file elements needing correction such asluminance element 316L may be decompressed to form correctable elementssuch as correctable element 320. Following step 321 alternate paths maybe used.

In a first embodiment of the present invention at step 325, correctableelement 320 and element 322 may be combined using data combiner 324 toform intermediate file 326. Intermediate file 326 has shared elementswith compressed image file 316. Correctable elements such as correctableelement 320 may be uncompressed awaiting correction and elements notrequiring processing such as element 322 may be combined in one or moreuncorrected intermediate format files such as uncorrected intermediatefile 326. Upon receipt of user color data such as display calibration orcharacterization data 38 of FIG. 1, uncorrected intermediate file 326may be processed at step 323 to correct correctable elements such ascorrectable element 320 according to display calibration orcharacterization data 38 which may be for a specific user only or it maybe a net correction file as discussed below. The result of step 323 maybe a corrected intermediate file such as corrected intermediate file328.

At step 327 corrected elements of corrected intermediate file 328 may becompressed according to the compression technique being used. Theresulting file composite corrected image file 332 is a luminancecorrected image file according to the compression technique being used.

Referring again to FIG. 1, correction of image file 52F for display mayinclude two or more alternate methods. In a first, display calibrationor characterization data 18D of the authoring display 18M may beincluded with or applied to an image file creating a master correctedimage file such as file 237 or uncorrected intermediate file 326 of FIG.10. Upon receipt of user display calibration or characterization data238 final correction of image file 237 may be accomplished. Thus file237 may be displayed on display 206 with corrections included fordisplay 208 and display 206. Alternatively, author display calibrationor characterization data 236 may be combined with user displaycalibration or characterization data 238 to create a net correction file239 that may be applied to any images authored on display 208 to achieveaccurate image display.

In a second embodiment of the present invention at step 325, correctableelement 320 may be corrected to form corrected element file 330. Asdiscussed above, upon receipt of user color data such as displaycalibration or characterization data 38 of FIG. 1, correctable element320 may be processed at step 323 according to display calibration orcharacterization data 38 which may be for the user only or it may be anet correction file as discussed.

At step 327 corrected elements such as corrected element file 330 may becompressed according to the compression technique being used. Compressedcorrected element file 334 may be combined with element 322 in combiner336 to form composite corrected image file 338. composite correctedimage file 332 and composite corrected image file 338 should yieldidentical images when displayed on display 22 of FIG. 1.

Starting from an original image file, this technique may also be appliedby originally compressing a portion of the image file. The uncompressedportion and the compressed portion and the authoring station colorcharacterization data may then be combined into an intermediate fileformat to permit fast correction and complete compression for transferto a user.

Image Preprocessing Sets

In another aspect, the present invention includes a technique fororganizing display devices into subsets according to theircharacteristics and thus limit image correction to a finite number ofperceptually uniform subsets. An image presented on display deviceswithin a subset should be indistinguishable to a user on all deviceshaving characteristics within the subset. Analysis of the relationshipbetween gamma, black-point and luminance for display devices such asmonitor 353 and monitor 361 demonstrated that within a gamma black-pointplane such as coordinate system 364 of FIG. 12, subset areas havinglimited variance luminance may be described.

Referring now to FIG. 11, in a currently preferred embodiment of thepresent invention network 350 includes two or more electronic devicessuch as devices 352, 354, 356, 358, 360 and 362. Electronic devices 352,354, 356, 358 and 360 further include display elements such as monitor353, monitor 355, display 357, display 359, monitor 361 and displaydevice 363 respectively. Display elements such as monitor 353, monitor355, display 357, display 359, monitor 361 and display device 363 may becharacterized using two or more parameters such as gamma black-point andluminance for CRT displays. Non-CRT display devices may use differentparameters.

Referring now to FIG. 12 coordinate system 364 includes characteristicaxes 366 and 368 illustrating the interrelationship betweencharacteristic 370 and 372 respectively. For conventional cathode raytubes displays such as monitor 353 coordinate system 364 has twocharacteristic axes 366 and 368 for characteristic 370 (gamma) and 372(black point) respectively.

One or more subset areas such as subset 374 may be used to identifyareas of luminance having nearly-indistinguishable image parameters forCRT display devices such as monitor 361 and display device 363. Subsetareas such as subset 374 and subset 376 may overlap. In a currentlypreferred embodiment of the present invention, subset overlapping isrequired to completely cover the characteristic space describing theimaging or display device. As characteristic 370 (gamma) and 372 (blackpoint) move away from origin 371, subset areas such as subset 378 mayinclude larger or smaller areas than subset areas closer to origin 371such as subset 374.

Display device parameters 370 (γ) and 372 (black point) may be obtainedfrom display device characterization as discussed above. Thus, when auser device 352 requests an image from a correction enabled server 354,server 354 may display parameters such as characteristic 370 (gamma) and372 (black point) from user display calibration or characterization data373 and may provide a pre-corrected image such as pre-corrected image375 according to which subset 374 the users display device may begrouped in. A server so enabled may store a finite number ofpre-corrected images such as pre-corrected images 380 to expeditefulfilling a user request for a corrected image according to the subsetof the users display device.

Referring now to FIG. 13, in an alternate embodiment of the presentinvention, a correction enabled server such as device 354 may use acombination of pre-corrected images in local storage to provide todisplay devices having subsets in area 386, pre-corrected images incentral or network storage for the smaller yet significant number ofdisplay devices having subsets in areas 384 and on-the-fly imagecorrection display devices having subsets in areas 382. Othercombinations of image correction and storage may be used. Distributionarea 386 may also be characterized in terms of one or more parameters ofdisplay 353, input or output device, or in terms of some other importantand useful characteristic used to subset display devices or images. Thedistributions need not be limited to a unidimensional characteristic,they may be multidimensional and encompass many display or imagingparameters.

In another embodiment of the present invention, information fromcharacterization data block 34 necessary to assign a user to a subset374 may be encoded into an image request such as image request 54 bybeing encoded in a URL or other request parameter. By encodingcharacterization data and by extension subset information onto the URLof a corrected image, the image may be cached.

Determining Input/Output Parameters of Any Display

Referring now to FIG. 14, in another aspect, the present inventionincludes a method and apparatus to establish the input/outputcharacteristics (I/O) and operating point such as point 392, and todetermine I/O curves of displays such as I/O curves 394 and 396, thatmay be applicable to any type of display technology such as display 357of FIG. 11. It can be used in conjunction with visual or instrumentalcharacterization or calibration methods. The method described in thisinvention is not limited to any particular display technology, but itwill be described using Liquid Crystal Display (LCD) technology as anexample. An application according to the present invention may run inconjunction with any type of display.

Referring now to FIGS. 15 and 16, an operating point determinationmethod according to the present invention includes two parts. The firstpart, data reduction 400, determines the appropriate subset oforthogonal basis vectors that describe the space of measured I/O curvessuch as I/O curve 394 along with the coefficients used to synthesize thecurves. In principle data reduction 400 need only be done once providingthe curves used in the analysis span the space of all possible I/Ocurves. It is this property that makes this a robust general method. Inpractice, data reduction 400 characterizes a large set of display I/Ocurves, or vectors, using a smaller set of orthogonal basis vectors. Ifeach I/O curve is represented by N input points, then there is apossibility that the space containing all measurable I/O curves isN-dimensional. Rarely is an I/O characteristic N-dimensional, usuallythe dimension is something less than N.

The second part of an operating point determination method according tothe present invention, data application 402, describes the determinationof a specific I/O curve such as I/O curve 394 for a users display suchas display 357. There are no constraints, both visual and instrumentalapproaches are possible.

Data Reduction

Referring now specifically to FIGS. 15 and 17, at step 401 datareduction according to the present invention tests displays such asdisplay 406 and measures screen luminance L, also called screenbrightness, as a function of known digital input values DV for neutralor near-neutral colors. Luminance versus digital input value data mayalso be compiled from existing data such as manufacturers data whereavailable. A plot such as graph 412 of measured luminance L, incandelas/m², versus DV yields a measured I/O transfer function such asI/O curve 410 of FIG. 18. Screen luminance may be determined using lightmeasuring device 408 which may be a spectroradiometer, calorimeter, orother form of light measuring device. Such measurement could also bedone on a relative basis by comparing the displayed luminance relativeto some reference, such as a “gray scale” or series of know areas ofreflectance. For I/O curves of specific display color primaries, eachprimary color would be displayed instead of the neutral color. Displaycolor primaries may be red, green and blue for a conventional RGBsystem, other systems may be used such as CMY, YUV or any other suitablecombination.

The number of input DV to be sampled should be sufficient to sample anycurvature of the I/O curves such as I/O curve 410. In a currentlypreferred embodiment of the present invention fifteen uniformly spacedinput DV levels have been used, but specific display devices mightdictate more or fewer levels. The actual number will depend on theinstantaneous slope such as slope S of I/O curve 410. A higher slopesuch as S₁ suggests more samples be used to adequately measure thecurve, and, with a lower slope such as S₂, fewer samples may be used.

A sufficient number of different display devices that span the range ofI/O characteristics of interest need to be measured or formulated fromuseful models. The measured data can be one device such as display 406measured at a multiplicity of display control settings, e.g. brightnessand contrast, or many different displays such as monitor 353, monitor355, display 357, display 359, monitor 361 and display device 363, othercombinations are possible.

At step 403 data 414 may be tabulated in a matrix format such as matrix416 where rows such as row 418 may correspond to each display such asdisplay 406 and/or display setting, and columns such as column 420 maycorrespond to input data DV. Matrix entries such as entry 422 may benormalized luminance values such as output luminance L. Data matrix 416may also be “inverted”, resulting in columns such as column 420representing the interpolated luminance values and the matrix entriessuch as entry 422 are the input digital values. Consistent with thespirit of the invention other normalization techniques may be used. In acurrently preferred embodiment of the present invention fifteen input DVvalues and twenty one different display conditions are used yielding a21 by 15 matrix.

Step 403 may also include data processing to include normalized displayluminance versus normalized DV for each display and/or display setting.Input data DV and output data L may be normalized by dividing by themaximum value in each case. This normalization yields a range of zero to1.0 for both input and output values.

Matrix 416 must be processed at step 405 before PCA. First, columnaverage 424 of each column 420 of data matrix 416 is determined. Thecolumn average is subtracted from each row 418 of data matrix 416. Thisnew matrix is called reduced matrix 426. A covariance matrix 428 iscomputed by pre-multiplying reduced matrix 426 by its transpose,transpose matrix 427. PCA is then performed on transpose matrix 427. Anysuitable conventional software programs may be used to carry out thecomputations.

At step 407, Principle Component Analysis (PCA) may be performed,(a.k.a.eigenvectors, characteristic vectors) on data matrix 416. The basic ideaof PCA is to represent the large collection of measured I/O curves orvectors, by a smaller set of orthogonal basis vectors. A weighted linearcombination of these basis vectors are then used to synthesize thecomplete set of I/O vectors.

In a currently preferred embodiment of the present invention after PCAat step 407, three vectors v₁, v₂ and v₃, plus a mean vector v_(m),accounted for about 99.88% of the variance in the different I/O curveshapes. This signifies that mean vector v_(m) plus some weighted linearcombination of basis vectors v₁, v₂ and v₃, may be used to synthesizeeach of the twenty one I/O curves used to generate the data quiteaccurately. In practice, the number of vectors can be more or less thanthree, depending on the variety of the measured or model curve shapes(the vector subspace) used in the analysis, and, the precision of thefit required.

Mathematically, I/O curve, Lj, at input, j, may be written as the linearcombination of the average vector and the three basis vectors as shownin equation 430.L _(j) ={overscore (v)} _(j) +a ₁ v _(1,j) +a ₂ v _(2,j) +a ₃ v_(3,j)  430

In equation 430 a₁, a₂ and a₃ are the vector weights and v₁, v₂ and v₃are the first three basis, or characteristic, vectors determined fromPCA in step 407. Since mean vector v_(m) and the three basis vectors v₁,v₂ and v₃, are fixed, only three scalar values a₁, a₂ and a₃ are neededto describe the complete I/O curve such as I/O curve 410. This is asignificant compaction of the data needed to describe the I/O curve.Without this representation it would take at least fifteen values, inour case, to describe each curve.

At step 409 three coefficients a₁, a₂ and a₃ in equation 430 aredetermined. Coefficients a₁, a₂ and a₃ are not necessarily related toany specific point on the I/O curve depending on original data matrix416. If data 414 were input digital values then there may be some simplerelationship between coefficients, a₁, a₂ and a₃ and some point on curve410. For a practical application coefficients a₁, a₂ and a₃ need to be“mapped” or connected to some measurable points on the I/O curve. Thesepoints can be determined using visual methods or instrumental methods.

For example, coefficients a₁, a₂ and a₃ may be determined as follows.For each of twenty one I/O curves initially measured or gathered, theDV's yielding 25%, 50% and 75% relative screen luminance may bedetermined by inverse linear interpolation of each I/O curve. That isthree DV's for each component channel such as red, green and bluechannels in a conventional RGB system. The other data set is the vectorcoefficients needed to synthesize the curves. Data set 434 now includesthree DVs, DV₂₅, DV₅₀, and DV₇₅, and three vector coefficients a₁, a₂and a₃, for each I/O curve 410 and the task is to relate DV andcoefficients.

In another aspect of the present invention, alternative DV sets may beused to more accurately characterize displays. DV₂₅, DV₅₀, and DV₇₅ maybe used for CRT displays and DV₃₃, DV₅₀, and DV₆₆ may be used for LCDdisplays. Other DV sets may be used successfully.

One technique is using polynomial regression to solve for b_(k) inequation 432.a _(k)=(b ₁ DV ₂₅ +b ₂ DV ₅₀ +b ₃ DV ₇₅)²  432

Other equations may be fitted by either regression or a variety of othercurve or function fitting operations. Another possibility is to use somefunctional form representing a physical model, or, use PCA again. Yetanother method might be to linearly or nonlinearly interpolate values,or interpolate a_(k) from a multidimensional table.

At step 411, data set 434 includes a set of three rectors v₁, v₂ and v₃,plus mean vector v_(m), and an equation for each coefficient a₁, a₂ anda₃ that relate the DV's determined from the matching by users or by aninstrument, to the coefficients, or weights, needed to synthesize orconstruct the curve. This needs to be done only once and may be put in adatabase 436 or stored in any other suitable storage system as shown inFIG. 16.

I/O Curve Construction

Once database 436 has been constructed a display I/O curve 410 for eachcolor channel or neutral gray may be created. The I/O curve thusconstructed can be written to a file, data set 434, computer memory 438,or otherwise stored for further use in system 440 according to dataapplication method 402 as part of a profile for color management orimage management. Image management can comprise any archiving of imagesor any form of image processing, either spatial or temporal.

Step 413 of data application 402 is to optimize the setup of the displaysuch as display 406. It is possible for users to misadjust the displaycontrols such as brightness control 444 and contrast control 442 so thehigh luminance levels are on shoulder 446 of I/O curve 410, and many ofthe low luminance levels are on toe 448 or lower curved part. Tooptimize operating point 450 of display 406 data for one or more setupscreens such as data 452 may be transmitted to user 404 to adjustcontrast control 442 and brightness control 444.

Referring now to FIG. 18, a setup screen 454 permits user adjustment ofdisplay 406 so there is a differentiation of two or more adjacent, orvery close, light (brightness) levels at high and low DV. Setup screen454 may include an array of patches or areas 456 and 458 either of grayor other display primary colors or color mixtures. Areas 456 and 458 maybe closely spaced in the highlights and shadow areas of the I/O curve.The user is instructed to adjust the “brightness” and “contrast”, or anyother display controls, to assure maximum color or luminance differencebetween the areas. This will help the user to operate the display offshoulder 446 or toe 448 of I/O curve 410 thereby increasing displaydynamic range.

For a conventional LCD display, the “brightness” knob generally controlsa fluorescent lamp or other light source behind the LCD and the“contrast” knob generally controls the operating point on the LCD.Therefore, the first adjustment should be the “contrast” to prevent theuser from operating the display on the shoulder of the curve. This maybe counterintuitive because it apparently causes a decrease in theoverall screen brightness. However, many LCD displays have a maximumluminance of about 50% greater than a bright CRT. A “bright” CRT mayhave a luminance of about 100 cd/m²—the sRGB standard is 80 cd/m²—whilemany of the better quality LCDs have a luminance value of about 150cd/m².

Area 456, at 75%, 66%, or any other suitable scale must not impinge ontoshoulder 446, and area 458 at 25%, 33% or any other suitable scale forexample, must not impinge into toe 448. Achieving an optimum displaysetting is not critical.

At step 415 user 404 is queried for inputs in order to determine thevalues for calculating the basis vector coefficients such as a₁, a₂ anda₃. Any combination of three or more points between 0% and 100% may besuitable.

In another embodiment of the present invention, three points from uservisual match data may be used to determine coefficients a₁, a₂ and a₃ asshown for example in Engeldrum & Hilliard U.S. Pat. No. 5,638,117. Sincethere are three vectors in the I/O curve synthesis, at least threepoints are need to estimate the three coefficients. With more or lessnumber of vectors describing the I/O curves, more or less points may beused. There is not necessarily a one-to-one correspondence between thenumber of vectors and the number of points used. One possibility is todisplay three, 25%, 50% and 75% halftone screens for each of the displaycolors, red, green, and blue with a number of continuous tone areasimmersed in the halftone background. This method is not limited to thethree standard so-called primary colors red, green and blue. In fact itis possible to construct a display using cyan, magenta and yellow thatmatch commercial printing standards in order to get a better match orother color systems may be used. This approach would work just as wellwith this display or any display that used one or more colorants orprimary colors. Also, the number of points and the percentage values canbe changed to increase precision, or accuracy of coefficientdetermination with any given display such as 33%, 50% and 66% or, black,33%, 50%, 66% and white. The user may select one of the embedded patchessuch as patch 460 that matches either in color or luminance (brightness)of the surrounding halftone 462. Since the DV for each displayed patchis known, these match values determine the DVs that match the 25%, 50%and 75% surround halftone screens. It is also possible to use aninstrument to make this comparison. Other arrangements of continuoustone and halftone areas are possible. For example it is possible to keepfixed a continuous tone patch such as patch 460 and make an adjustmentof the surrounding halftone such as halftone 462 so there is matchbetween the patch and the halftone.

In still another embodiment of the present invention, a series ofpatches 464, or images, of known relative DV surrounded by a halftone462 of known fractional area is presented on a screen 454. An observeris asked to select one of the patches that matches the halftonebackground. This matching process may then be repeated for two or moreother surround halftone values yielding at least three DV-relativeluminance pairs. Fractional areas of 25%, 50% and 75% are useful butother values may be better in different situations.

In still another embodiment of the present invention a radiation orlight measuring device such as light measuring device 408 may be usedand display 406 may be controlled by a computer 466 to present allpossible light (color) values in an automatic method. Computer 466 maybe programmed to perform a search to find a displayed area 458 that isclosest in luminance to a reference luminance, say 75% of the maximumluminance. For popular eight bit systems this does not mean that all 256levels need to be presented. A binary search method would be very rapid,only requiring the display of patches equal to the number of bits ofradiant resolution. For an 8 bit display this would required the displayof eight areas, at most, to find the closest input value to the 75%reference value. This process can be repeated for as may values or matchpoints as necessary. Other search methods can be used, for example, someform of table lookup.

At step 417 vector coefficients a₁, a₂ and a₃ may be calculated fromregression equation 432, or from a lookup-table or tables, using DVs asindependent variables, or possibly the relative luminance obtained bymaking a halftone-patch match. Other forms of database or datacalculations may also be used.

At step 419 equation 430 may be used to calculate the display I/O curvesuch as I/O curve 410 at each input DV point, j. As in the aboveexample, original data set 414 sampled the input (DV) at fifteen points.This is usually not sufficient for specifying a display profile havingan 8 bit input having 256 levels. To compute all 256 or more, points ofthe I/O curve, several possibilities are available. If the basis vectorssuch as vectors v₁, v₂ and v₃ are smooth functions of the input DV theycan be fit by polynomials or other continuous functional forms. Someform of interpolation is also a method that may successfully be applied.Since the basis vectors are fixed, these need to be interpolated onlyonce and can be stored. In the case of the functional form for the basisvector coefficients equation 430 now becomes equation 468 below:

 L(DV)={overscore (v)}(DV)+a ₁ f ₁(DV)+a ₂ f ₂(DV)+a ₃ f ₃(DV)  468

where f_(x)(DV) may be the polynomials representing the basis vectorsv₁, v₂ and v₃ and 0≦DV≦1. A polynomial representation, or otherfunctional representation of the mean vector may also be used.

Reconstructed I/O curve 470 may “overshoot” and/or “undershoot” theactual curve 410. This means that the relative luminance exceeds 1.0, orgoes negative. The simple fix is to clip I/O curve 470 to 1.0 the firsttime it exceeds 1.0, and clip to 0 the first time it goes negative. Bychecking the 8 bit LUT from the middle of the curve toward the “ends”,one can readily determine the first “overshoot” and “undershoot”conditions. Other methods are possible, such as locally altering thetransition of the I/O curve at the zero and one points.

In the process of determining a visual match a user may select a patch472 that generates an unrealistic coefficient a_(u). There are many waysto deal with this, but a simple way is to ignore basis vectors v₁, v₂and v₃ and just report mean vector v_(m). Depending on the basisvectors, the mean vector as a default I/O curve may be adequate for mostpurposes.

Flat Panel Characterization and Calibration

In another aspect of the present invention, a method is provided tocharacterize a flat panel display device. The method provides aparametric function that is an approximation for the Tone ReproductionCurve (TRC) of LCD (and other flat panel) monitor screens, and acharacterization method to find the parameters from a limited amount offunction points.

Optimally the characterization method should fulfill the followinggoals:

-   -   Also give acceptable results for CRT displays.    -   Only need two calibration points.    -   Have stable behavior for input variability.        The proposed function is the second order expression 600:        $\begin{matrix}        {L = \left\lbrack {1 - {\cos\left( \frac{\pi\quad x}{2} \right)}^{p}} \right\rbrack^{r/1.75}} & (600)        \end{matrix}$        with parameters (powers) p and r. The constant 1.75 is chosen in        a preferred embodiment to result in an approximation of unity        for p and r both equal to 1. For p equal to 1, this function        will closely approximate a first order power function, and thus        constitute a model for CRT displays. (Black point calibration is        considered a separate issue in this context.) For r equal to        1.75 and p equal to 2, the function will have a pure half period        cosine shape, which is characteristic for a class of Flat Panel        Displays.

For the above mentioned characterization method, we must find a methodto estimate the two parameters from two calibration values. From currentexisting methods it is advantageous to take the input values resultingin output values of 0.333 and 0.667 as these points. Thus, in apreferred embodiment, a viewer of the display is presented with “known”images evincing ⅓ and ⅔ respectively of the display device's maximumluminance, and asked to select the best match to each known image fromamong multiple comparison images that are comprised of various colorinputs. Alternatively, the viewer may be presented with a slider thatcan be moved to adjust the color of a single comparison image to matchthe known image. Based upon the viewer selection, the inputs necessaryto produce ⅓ and ⅔ luminance on the given display device are obtained.Alternatively, more accurate methods may be employed to obtain thesevalues, such as using a spectrometer to match the known images.

This gives us two known values x₁ and x₂ for the input voltage,resulting in two non-linear equations with the unknown parameters p andr:${\left\lbrack {1 - {\cos\left( \frac{\pi\quad x}{2} \right)}^{p}} \right\rbrack^{r/1.75} - \frac{1}{3}} = {{{0\left\lbrack {1 - {\cos\left( \frac{\pi\quad x}{2} \right)}^{p}} \right\rbrack}^{r/1.75} - \frac{2}{3}} = 0}$Because we have this analytical description, we can use a twodimensional variation of the Newton-Raphson method. Let the above systembe represented by: F_(i)=0. Then we can form the Jacobian matrix ofpartial derivatives:${J_{ij} \equiv \frac{\partial F_{i}}{\partial q_{j}}},$where q₁=p, q₂=r. Starting with a first approximation vector q, we canfind fast converging new solutions by:q _(new) =q _(old) −FJ ⁻¹,

This method is very sensitive for the suitability of the firstapproximation. If we are not close enough to the required solution, theiteration can diverge significantly. This is especially harmful withfunctions that can easily give complex results. For the preferredembodiment, we chose the following solution for these problems:

-   -   Replace the parameters by exponential expressions, thus making        sure to get safe positive powers for all real values in the        search domain.    -   The first approximation is found by bilinear interpolation in a        two dimensional table.    -   After each iteration a test is made whether we are strictly        closer to zero. If not, we can progressively backtrack along the        direction given by the inverse Jacobian, until this prerequisite        is fulfilled.        This results in a guaranteed stable solution for the system of        equations. Applying this calibration method to CRT gamma-like        curves results in an approximation, which never exceeds a        deviation of two delta-E units. For a set of measured Flat Panel        curves the average (over the different displays) maximum (over        the luminance scale) delta-E was 9. The ‘average average’        delta-E was 1.5.

FIG. 21 shows various measured TRCs for a number of different flatscreens. As can be seen, the family of flat screens tends to exhibitgenerally S-shaped curves that are very well approximated by thefunction of two powers p and r described above. In addition, however,this function also provides very good results for CRT type screens thathave typically been modeled by a function of a single power, gamma:x^(γ). Thus, obtaining two input values from a CRT screen by the methoddescribed above will result in parameters p and r (typically p=1) thatwill allow the expression 600 to very closely model the TRC of the CRT.Thus, a system using the expression 600 will be able to characterize andcolor correct images for both CRT and flat screen monitors using thesame software and methodology, as well as other monitors including, butnot limited to, organic light emitting diode (LED) screens,electroluminescent displays, and plasma or gas-discharge screens.

Those skilled in the art will realize that this representation of theTone Reproduction Curve or Input/Output curve of a display can beexpanded to include such things as the absolute luminance or radiance ofthe display, ambient lighting and the ideal of a “black point” orcutoff.

The addition of a constant to the proposed function suffices to capturethe ambient light reflected either directly from the screen or incidenton the screen in a projection display configuration. The followingequation (601) provides an example. $\begin{matrix}{L = {{{k1}\left\lbrack {1 - {\cos\left( \frac{\pi\quad x}{2} \right)}^{p}} \right\rbrack}^{r/1.75} + {k2}}} & (601)\end{matrix}$In this example k2 is a parameter that provides for the ambient and k1is a parameter that can be used to scale the function for absoluteluminance, radiance, or irradiance.

A “cutoff” or “black point” can be incorporated by adding a thirdconstant, k3. $\begin{matrix}{L = {{{k1}\left\lbrack {1 - {{\cos\left( \frac{\pi\quad x}{2} \right)}^{p}{k3}}} \right\rbrack}^{r/1.75} + {k2}}} & (602)\end{matrix}$If the value inside the brackets is negative, due to a negative value ofk3, then the value is set to zero.

Of course, this function can also be used to describe the relationshipof screen luminance or illuminance and the input to a video or cinemaprojector or some form of image projection system.

Although the relative luminance values of ⅓ and ⅔ were described in thepreferred embodiment, those skilled in the art will appreciate thatother values can be used with equal success. The relative luminancevalues of ⅓ and ⅔ are advantageously selected for LCDs because thecolors of continuous tone color images are formed by mixing three linesof colors, and thus ⅓ and ⅔ luminance can be relatively accuratelyrepresented by a ratio of 1 to 2 and 2 to 1 lines, respectively.

Although the method of solution described above assumes that the valuesx1, and x2, are without error, it is possible that in some applicationsthere will be error in these values. It will be apparent to thoseskilled in the art that other methods of solving for the parameters arepossible, such as nonlinear least squares solutions, maximum likelihood,and other criteria optimization schemes.

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications in the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asset forth in the following claims.

1. A method for predicting the color output of a display device as asecond order function of the input voltage to the device, comprising:obtaining the input voltage value required to display on the device eachof at least two images having known luminance values; and solving thefunction as at least two simultaneous equations based on the obtainedinput voltage values to calculate the two powers for the function todescribe the device tone reproduction curve.
 2. The method of claim 1,wherein obtaining the input voltage values comprises: obtaining inputvoltage values required to display on the device images having knownluminance values from the interaction of a viewer with the images havingknown luminance values on the device.
 3. The method of claim 2, whereinobtaining the input voltage values from the interaction of a viewer withthe images having known luminance values on the device comprises:presenting a plurality of continuous tone images for matching with eachimage having a known luminance value, each continuous tone imagecorresponding to a different pre-selected input voltage; and for eachinput voltage value, using the input voltage of the continuous toneimage selected by the viewer.
 4. The method of claim 1, whereinobtaining input voltage values comprises: measuring the color output ofthe device when the images having known luminance values are displayedon the device.
 5. The method of claim 4, wherein measuring the coloroutput of the device comprises: measuring the color output of the devicewith a spectrometer.
 6. The method of claim 1, wherein solving thefunction comprises: solving the function$L = \left\lbrack {1 - {\cos\left( \frac{\pi\quad x}{2} \right)}^{p}} \right\rbrack^{r/1.75}$ wherein L is the luminance, x is the obtained input voltage value, andp and rare the two power parameters of the function.
 7. The method ofclaim 6, wherein solving the function further comprises: scaling thefunction to account for one or more factors selected from the group offactors including black point cutoff, ambient light, absolute luminance,absolute radiance, and absolute irradiance.
 8. The method of claim 1,wherein obtaining the input voltage values comprises: obtaining theinput voltage value required to display on the device each of an imagehaving a luminance value of ⅓ of the maximum device luminance and animage having a luminance value of ⅔ of the maximum device luminance. 9.The method of claim 1, further comprising: adjusting characteristics ofan image in accordance with the device tone reproduction curve fordisplay on the device.
 10. The method of claim 9, wherein adjustingcharacteristics of the image comprises: adjusting characteristics of theimage in accordance with differences between the device tonereproduction curve and a predetermined tone reproduction curve.
 11. Themethod of claim 1, wherein the display device is selected from the groupincluding liquid crystal displays (LCD), plasma screens, gas-dischargescreens, cathode ray tube (ORT) monitors, light emitting diodes (LED),organic LED, and electroluminescent displays.
 12. The method of claim 6,wherein the display device is a cathode ray tube monitor and solving thefunction comprises: setting the power parameter ρ equal to
 1. 13. Amethod for displaying color images on a display device, comprising:solving a second order function for predicting the device tonereproduction curve as at least two simultaneous equations based on inputvoltage values required to display images on the device having knownluminance values; adjusting characteristics of the images in accordancewith the tone reproduction curve described by the second order function;and providing the adjusted images for display on the device.
 14. Themethod of claim 13, wherein solving the function comprises: calculatingthe two powers for the function to predict the device tone reproductioncurve.
 15. The method of claim 14, further comprising: obtaining inputvoltage values required to display on the device images having knownluminance values from the interaction of a viewer with the images havingknown luminance values on the device.
 16. The method of claim 15,wherein obtaining the input voltage values from the interaction of aviewer with the images having known luminance values on the devicecomprises: presenting a plurality of continuous tone images for matchingwith each image having a known luminance value, each continuous toneimage corresponding to a different pre-selected input voltage; and foreach input voltage value, using the pre-selected input voltage of thecontinuous tone image selected by the viewer.
 17. The method of claim14, wherein obtaining input voltage values comprises: measuring thecolor output of the device when the images having known luminance valuesare displayed on the device.
 18. The method of claim 17, whereinmeasuring the color output of the device comprises: measuring the coloroutput of the device with a spectrometer.
 19. The method of claim 14,wherein solving the function comprises: solving the function$L = \left\lbrack {1 - {\cos\left( \frac{\pi\quad x}{2} \right)}^{p}} \right\rbrack^{r/1.75}$ wherein L is the luminance, x is the obtained input voltage value, andp and r are the two power parameters of the function.
 20. The method ofclaim 19, wherein solving the function further comprises: scaling thefunction to account for one or more factors selected from the group offactors including black point cutoff, ambient light, absolute luminance,absolute radiance, and absolute irradiance.
 21. The method of claim 14,wherein solving the function comprises: solving the function based onthe input voltage value required to display on the device each of animage having a luminance value of ⅓ of the maximum device luminance andan image having a luminance value of ⅔ of the maximum device luminance.22. The method of claim 19 wherein adjusting characteristics of imagescomprises: adjusting characteristics of each image in accordance withdifferences between the device tone reproduction curve and apredetermined tone reproduction curve.
 23. The method of claim 14,wherein the display device is selected from the group including liquidcrystal displays (LCD), plasma screens, gas-discharge screens, cathoderay tube (CRT) monitors, light emitting diodes (LED), organic LED, andelectroluminescent displays.
 24. The method of claim 19, wherein thedisplay device is a cathode ray tube monitor and solving the functioncomprises: setting the power parameter ρ equal to 1.